Method for depositing high performing electrochromic layers

ABSTRACT

An electrochromic coating is produced by adding an organic moiety to a solution of an electrochromic precursor, said organic moiety having a decomposition temperature greater than, or a vapor pressure sufficiently low at, the temperature at which solution solvent is removed, such that said organic moiety remains integral with the electrochromic precursor coating on said substrate after said solvent evaporates, and said organic moiety having a decomposition temperature lower than, or a vapor pressure sufficiently high at, the temperature at which said electrochromic precursor coating is converted to an electrochromic coating that said moiety is substantially removed from said coating before or during said conversion.

BACKGROUND OF THE INVENTION

This invention relates to methods of making optical quality coatings ofinorganic oxides on glass or equivalent substrates, such as metals orceramics. These inorganic oxide coatings are primarily electrochromic,but may also have other desirable properties such as electricalconductivity and anti-reflection. Electrochromic films undergoreversible coloration induced by an applied electric field or current.These inorganic electrochromic layers can be broadly classified intothose that color cathodically due to the double injection of electronand cation (Group VI-B oxides such as WO₃ and MoO₃) and those that coloranodically (Group VIII oxides such as IrO₂, Rh₂ O₃, NiO and CoO).Electrochromic coatings are used in information display devices, solarcontrol windows and light modulators.

In a typical electrochromic device, the electrochromic coatings are incontact with an electron conductor and an ion conductor. The electronconductor can be a paste or coating on a substrate, or a stand-alonemonolith. The ion conductor, or electrolyte, may be a liquid, paste orsolid. Electrochromic coatings work by the injection or ejection of ionsand electrons between the electron conductor and the ion conductor.

The most common way to deposit electrochromic films is by vacuumtechniques, typically evaporation or sputtering. Non-vacuum techniquessuch as anodization and atmospheric chemical vapor deposition are alsoused. Evaporation deposition and sputter coating require a high vacuum.While such techniques require expensive capital equipment, they havebeen commonly used to produce electrochromic coatings.

Three similar non-vacuum coating techniques which have been used to alimited extent for electrochromic coatings are dip coating, spraycoating and spin coating. These wet chemical solution coating techniquesoffer the advantage of being less capital intensive and thus lessexpensive. Dip coating, as an example, is commonly used to coat glasswith SiO₂. This process involves lowering a glass substrate into asolution containing an appropriate precursor of the desired oxide. Spincoating and spray coating are similar to dip coating except that insteadof dipping the glass, the precursor solution is applied to the glass,which is spun to spread the coating out, or is sprayed onto heatedglass.

It is desirable to be able to achieve amorphous electrochromic coatingsthat are resilient to ionic intercalation such as occurs when ions suchas hydrogen ions, lithium ions, sodium ions and the like are insertedand removed during electrochromic coloring and bleaching. It is alsodesirable to achieve amorphous electrochromic coatings that are durableto mechanical abrasion, chemical attack (as for example by acidicelectrolytes) and the like.

One way to achieve such coatings is to consolidate the amorphousstructure by heating to high temperatures and in some cases evencrystallizing the tungsten oxide. Unfortunately, the less expensive wetchemical deposited films above a certain thickness (2,500 angstroms orthereabouts) usually crack during consolidation and crystallization dueto volumetric shrinkage, among other effects. While thinner wet chemicaldeposited films are not as likely to crack during consolidation orcrystallization, they do not color as deeply as thicker films.

In addition, when crystallization occurs in wet chemical depositedfilms, the coloration kinetics of the film becomes slower and/or itsability to color reduces. This is due to a decreased ionic diffusion andmay also be related to the potential loss of coloration sites.

SUMMARY OF THE INVENTION

In this invention, the difficulties indicated above are overcome byusing wet chemical deposition to form electrochromic films with a porousmicrostructure which allows fast diffusion of ions and also providesmany surface sites for coloration. This is accomplished by coating thesubstrate with an electrochromic precursor solution which includes aremovable moiety. The solvent, moiety and electrochromic precursor areselected such that the solvent is preferentially removed from thecoating in a first removal step, leaving a matrix film of theelectrochromic precursor and the moiety. In turn, the moiety ispreferentially removed from the matrix in a second removal step priorto, or during, conversion of the electrochromic precursor to anelectrochromic material.

This invention encompasses methods to allow film thickness to be muchhigher without physical degradation (such as cracking and lifting offthe substrate). Higher film thickness enables higher optical attenuationfrom the devices, resulting in reduced light transmission and bettercontrast. Because the films of this invention have a porous structure,even when fired at elevated temperatures such as 150° C., or higher,volumetric strain also is reduced when ions diffuse through them duringthe coloration and bleach process, thus extending their cyclability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 plots a humid atmosphere regimen for pretreating precursorsolution coatings on conductive substrates.

FIG. 2 plots after-firing film density against variation in film moietyconcentration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention contemplates various types of electrochromicprecursors and electrochromic coatings, and, accordingly, various typesof conversion processes, the preferred embodiment electrochromicprecursor is one which is converted to an electrochromically-activemetallic oxide at elevated temperatures. Similarly, while a variety oftechniques may be used to remove solvent from the precursor/moietysolution, the preferred embodiment first removal step involvesevaporation in vacuum and/or heating to a temperature sufficiently highto remove solvent, but below the temperature at which the moiety mightbe removed and below the temperature at which the electrochromicprecursor would be converted to an electrochromically-active metallicoxide. Similarly, the preferred second removal step comprises furtherheating of the precursor/moiety matrix to the precursor conversiontemperature.

To assure the ultimate proper formation of a porous microstructure, themoiety must substantially remain in the as-deposited coating while thesolvent is removed therefrom. Thus, the moiety must have a decompositiontemperature sufficiently high, or vapor pressure sufficiently low, atthe temperature and/or vacuum at which the precursor solution solventevaporates, so that the moiety molecules are integral with the precursorcoating which is formed as the precursor solution solvent evaporates.The moiety and precursor form an immobile matrix film after the solventhas evaporated.

On the other hand, the moiety must have a decomposition temperaturesufficiently low, or a vapor pressure sufficiently high, at thetemperature at which the electrochromic precursor is converted to anelectrochromically-active metallic oxide, so that the moiety issubstantially removed by decomposition or evaporation or both prior to,or generally during, conversion of the electrochromic precursor to theelectrochromically-active metallic oxide.

A substrate with a conductive surface is preferably dipped into theprecursor solution and withdrawn at a rate sufficient to give a coatingof the desired thickness over the conductive surface, though spray orspin coating or equivalent wet chemical deposition means can be used inthe broader aspects of the invention. The coating is then dried at roomtemperature or elevated temperature to remove the solvent and ispreferably fired in an oven to complete the condensation and hydrolysisto yield an electrochromic oxide coating having exceptionalelectrochromic properties and stability.

Various types of precursor solutions for electrochromically-activemetallic oxides are known and are useable in the broader aspects of thisinvention. The preferred metallic oxide precursor solution is preparedby reacting a transition metal with a solution of a mixture of hydrogenperoxide and an organic acid, or sequentially reacting the hydrogenperoxide with the metal and then reacting the product with an organicacid. The reaction product is filtered and the filtrate preferably isevaporated to dryness under reduced pressure. The resulting transitionmetal-peroxy acid product (liquid or powder) is then reacted at roomtemperature by mixing with a lower carbon alcohol to form a transitionmetal-peroxyester derivative. This solution can be used for dipping, butpreferably the transition metal-peroxyester derivative is isolated byremoving excess alcohol under vacuum. This peroxyester-transition metalderivative, when dissolved in a suitable carrier solvent, constitutesthe preferred dipping solution.

The reaction between the transition metal, hydrogen peroxide and organicacid is conducted at a temperature controlled at about -10° C. to about12° C. The ingredients are then allowed to react at that temperaturefrom about 16 to about 26 hours. After the reaction, the reactionproduct is filtered to remove the solids from the filtrate. The filtrateis then refluxed for about 10 to about 18 hours at from about 45° C. toabout 60° C. and then refiltered. This second filtrate is usually driedto recover the transition metal-peroxy acid product as a powder. Thetransition metal-peroxy acid product (liquid or powder) is then reactedat room temperature with an alcohol in a flask loaded with a stir bar.The reaction temperature can vary from around 22° C. to around 55° C.,and the reaction time can vary from about 30 hours to about one-halfhour, depending on temperature. Preferably, the peroxyester-transitionmetal derivative is stored long-term at below 10° C.

Typical transition metals which can be used as starting materialsinclude those of tungsten, molybdenum, manganese, chromium, rhenium,iridium, nickel and others. The peroxyester derivatives of thesetransition metals can be used separately or mixtures of them can be usedto form electrochromic coatings. In addition, the peroxyesterderivatives of these transition metals can be mixed with peroxyesterderivatives or peroxy acid products of other transition metals or ofnon-transition metal oxides, such as silica and titanium dioxide, toform electrochromic coatings. Mixtures of transition metal peroxyesterderivatives form electrochromic coatings having enhanced colorefficiency. Mixtures of a transition metal-peroxyester derivative and anon-transition metal-peroxy acid product or a non-transitionmetal-peroxyester derivative form electrochromic coatings havingenhanced durability.

The electrochromic precursors can be prepared in several different waysto yield mixed electrochromically-active metallic oxide coatings. Asecond metal can be reacted with hydrogen peroxide and an organic acidto form a second transition metal-peroxy acid product, which can then bemixed with a peroxyester-transition metal derivative to form aperoxyester-transition metal/second metal-peroxy acid mixture.Alternatively, the second metal-peroxy acid product can be reacted witha lower carbon alcohol to form a peroxyester-second metal derivativeprior to mixing with another peroxyester-transition metal derivative.

The peroxide used for forming the peroxy acid product is typicallyhydrogen peroxide. It is contemplated, however, that other peroxides canbe utilized such as peroxyacetic acid and 3-chloroperoxybenzoic acid.

Organic acids are used for forming the peroxy acid product. While avariety of organic acids are operable, the most preferred organic acidsare acetic acid and propionic acid.

The preferred lower carbon alcohols added to the transition metal-peroxyacid product are methanol, ethanol, propanol, isopropanol, and mixturesthereof, although butyl and pentyl alcohols are broadly operable. Thesesame alcohols can be used as carrier solvents for the precursorsolution. Other carrier solvents include tetrahydrofuran, diethyl etherand other equivalents thereto. They should evaporate readily, i.e. attemperatures lower than 100°-120° C. such that an electrochromicprecursor and moiety matrix coating are readily formed by evaporatingthe carrier solvent.

Preferred removable moieties are organic compounds such as lower carbonacids such as oxalic, malonic, succinic, glutaric and adipic acids,lower carbon bases such as formamide and higher boiling lower carbonalcohols such as glycerol. The organic moiety must have a decompositiontemperature sufficiently greater than the temperature at which thecarrier solvent is removed, or must have a vapor pressure sufficientlylow at the temperature at which the carrier solvent is removed, that itsmolecules remain integral with the precursor coating which is formed asthe solvent evaporates to form a precursor/organic moiety matrix and toassure the ultimate proper formation of a porous microstructure. It isalso necessary, however, that the organic moiety have a decompositiontemperature less than or equal to the "firing" or "curing" temperatureat which the precursor coating is substantially converted to anelectrochromic coating, or have a vapor pressure sufficiently high at orbelow the conversion temperature so that decomposition or evaporation ofthe organic moiety occurs prior to or concurrently with theestablishment of the metallic oxide structure.

The metallic oxide structure is typically established at temperatures offrom about 150° C. to about 400° C. with 250° C. to 400° C. being mosttypical. In general, it is preferable that the organic moiety, if solid,should have a melting point of 50° C. and, preferably, at least 100° C.,and if non-solid, have a boiling point at 760 torr of at least 120° C.The selection of a low carbon content organic moiety is importantespecially for the moieties which decompose to non-volatile residues,because entrapment in the oxide structure of too much foreign materialsuch as carbon or equivalent residues can affect the optical quality,disturb the refractive index, and cause the deterioration of a varietyof physical, chemical or functional properties of the electrochromiccoating. Moieties of six carbon atoms or less are most preferred.

Examples of organic moieties which are operable, along with theirrespective boiling points or melting points are set forth below:

    ______________________________________                                        ORGANIC MOIETY    MELTING POINT                                               ______________________________________                                        Oxalic acid       190° C.                                              Oxalic acid dihydrate                                                                           104-106° C.                                          Malonic acid      135-137° C.                                          Succinic acid     187-189° C.                                          Glutaric acid     95-98° C.                                            Adipic acid       152-154° C.                                          Ethylene diamine tetra-                                                                         250° C.                                              acetic acid                                                                                     BOILING POINT                                               ______________________________________                                        Glycerol          290° C. at 760 torr                                  Formamide         210° C. at 760 torr with                                               partial decomposition                                                         at 180° C.                                           ______________________________________                                    

For electrochromic applications, the substrate which is to be coatedwith the organic moiety containing electrochromic coating itself shouldhave a conductive surface. Good conductivity is important to achieving afast response time and uniform coloration in the electrochromic coating.In the case of glass or ceramic substrates, such conductivity at thesurface can be achieved by applying a conductive coating prior to theelectrochromic coating. Preferably, this coating has a sheet resistanceof less than 10 ohms per square.

In glass applications where one should be able to see through the glassand the conductive coating, it is important that the conductive coatingbe very thin so that light transmission is not excessively inhibited. Inorder to achieve a sheet resistance of less than about 10 ohms persquare and still have a coated piece of glass with a light transmissionof 85% or greater, the material used to create the conductive coatingshould have a specific resistivity of less than about 8×10⁻⁴ ohmcentimeters, and most preferably, less than about 4×10⁻⁴ ohmcentimeters. Indium tin oxide coatings (ITO) can be achieved which havea specific resistivity of about 2×10⁻⁴ ohm centimeters. ITO is the mostpreferred coating material for glass, ceramic or equivalent substrateswhich themselves are nonconductive. Alternative transparent conductivecoatings which could also be used are doped oxides of tin and zinc, andcadmium stannate.

The electrochromic precursor is dissolved in the carrier solvent at aconcentration of from about 5% to about 60% by weight. The molar ratioof organic moiety to electrochromic precursor preferably falls within arange of from about 0.1:1 to about 2:1, and most preferably from about0.3 to about 1:1. While the corresponding weight percent of organicmoiety in solution will vary with molecular weight of the moiety, itwill typically be less than 10 weight percent and more typically betweenabout 2 and 5 weight percent.

Upon incorporation of the organic moiety to form the working solution,the coating can be deposited on the substrate by spin, dip or equivalentmeans. In the case of dipping, the substrate is dipped into the solutionand slowly withdrawn. The thickness of the coating is a function of thewithdrawal rate and the viscosity of the dipping solution as shown inEquation 1. ##EQU1## Where:

t=coating thickness

V_(s) =withdrawal rate

n=viscosity

d=coating density

g=gravitational constant

The coating thickness can be varied over a wide range merely byregulating the withdrawal rate, dipping solution concentration and theviscosity of the dipping solution. Removal rates of between about 8 to50 centimeters per minute and solution concentrations between about 10and 15 weight percent yield coatings of from 200 to 10,000 angstroms.

The as-deposited coating is converted to an electrochromically-activecoating by removal of the volatile solvent to form a precursor/moietymatrix, followed by removal of the moiety and hydrolysis, condensationand similar reactions to convert the precursor to anelectrochromically-active film. Decomposition or evaporation of theorganic moiety occurs at a temperature higher than that required forremoval of the volatile solvent but at a temperature less than or equalto the temperature at which the oxide structure is substantiallyestablished. The coated substrate is gradually heated from roomtemperature to the conversion temperature (about 100° C. to 250° C. orthereabouts, if an amorphous end product is desired, above about 350° C.if a crystallized end product is desired) at a rate of 10° C. per minuteor lower, preferably about 5° C. per minute. Firing is preferablyallowed to proceed for about 15 to about 120 minutes. Different metallicoxide coatings will require differing firing conditions, as will beappreciated by reference to the examples herein. Alternatively, theas-desposited coating can be placed in a vacuum, or exposed to a flow ofinert gas such as nitrogen or argon or the like, to remove the carriersolvent, followed by heat treatment at elevated temperatures toultimately form the desired oxide structure.

It has been surprisingly found that an especially enhanced networkformation can be obtained by first gradually heating the coatedsubstrate from room temperature to about 100° C. in a humid atmospherewhere the relative humidity throughout the heat treatment is at least20% relative humidity (%RH). The substrate is brought to thistemperature at a rate of 10° C. per minute or lower, preferably about 5°C. per minute, and held at about 80° C. to 120° C., with about 100° C.preferred for about 30 to 120 minutes with about one hour preferred.After completion of this step, the coated substrate is allowed to coolback to room temperature and is then gradually heated in ambientatmosphere as described above to the desired "curing" or "firing"temperature (from about 150° C. to about 400° C.) at which theelectrochromic precursor is converted to an electrochromic metallicoxide coating. We find that such a curing regimen incorporating apretreatment step of purposefully preheating in a substantially humidatmosphere enables ultimate achievement in a single dip or slurryapplication of high performing, crack-free, high quality coatings ofthickness 2,500 angstroms or greater.

Although use of organic moieties to control microstructure in sol-gelcreated bulk structures is known, the benefits to electrochromicperformance in thin-film electrochromic coatings achieved by the use ofthe organic moieties of desired high decomposition temperatures and lowcarbon content has hitherto not been known in the art. Nor have thebenefits of a pretreatment step of drying in a humid atmosphere incombination with use of organic moieties in forming electrochromicthin-film coatings been previously known in the art.

It has also been found that the electrochromic layers deposited inaccordance with the present invention can be protected againstenvironmental degradation, as for example by exposure to acid and basicmedia, by using a tantalum pentoxide overlayer. The tantalum pentoxideoverlayer also serves as an electrolyte layer for protons between twoelectrochromic materials. Similarly, a lithium ion conducting overlayercoating could be deposited on top of the WO₃ layer for thoseelectrochromic devices that work by reversible insertion/extraction ofLi⁺ ions.

The following Examples illustrate the preparation of preferred coatingsolutions:

EXAMPLE 1

Dipping solutions were prepared as follows: Forty (40) milliliters ofdeionized water was placed in a one liter pear-shaped flask loaded witha stir bar. The flask was placed in an ice bath with 800 milliliters ofa 1:1 solution of hydrogen peroxide (31 volume percent) and glacialacetic acid. When the mixture equilibrated to the bath temperature, 65grams of tungsten metal was added to the mixture and the mixture wasallowed to react for 24 hours and the remaining solids removed byfiltering. This liquid was refluxed for 18 hours at 55° C. The solutionwas dried under vacuum to recover powdered tungsten peroxy acid product.Fifty (50) grams of this powder was reacted with 250 milliliters dryethanol. The solution was filtered and the solids isolated by removingsolvents under reduced pressure. The resulting tungsten peroxyesterderivative is a yellow powder which is soluble in ethanol and decomposesat temperatures greater than 10020 C. Thirty-one (31) grams of thetungsten peroxyester derivative was redissolved in 70 milliliters ofethanol to prepare a dipping solution. Ethanol has a boiling point ofabout 78° C. at 760 torr.

The dipping solution was divided in half and to one half was added theorganic moiety oxalic acid dihydrate in five weight percent. Oxalic aciddihydrate has a melting point of about 104° C. to 106° C. Indium tinoxide (ITO)-coated substrates were lowered in air at room temperatureand atmosphere into each of the two solutions respectively preparedabove and coated by withdrawing at different speeds. For the solutionwith oxalic acid dihydrate, the withdrawal rate was four centimeters perminute, and for the solution without oxalic acid dihydrate, the rate waseight centimeters per minute. The coatings were 2,000 angstroms thick. Apretreatment step of firing in a substantially humid atmosphere was thenconducted on the coated substrates in accordance with thetemperature/relative humidity versus time regimen schematicallyillustrated in FIG. 1. Both of the samples were then cut into twohalves, one of which was fired to 250° C. (amorphous), and the other to350° C. (crystalline) in an oven in ambient atmosphere for about 60minutes. The coatings were colored in 0.01N H₂ SO₄ using -0.4 volts tocolor and +0.4 volts to bleach with a Ag/AgCl reference electrode. Thecolor kinetic data at 550 nanometers for the different coatings istabulated in Table 1 (550 nanometers is the wavelength at which thehuman eye is most responsive). As can be seen from the table, theaddition of oxalic acid dihydrate to the dipping solution enhances therate of coloring and bleaching for both the amorphous and crystallinecoatings.

                  TABLE 1                                                         ______________________________________                                        Tabulated data for the coloring kinetics at 550 nanometers                    of WO.sub.3 films from ethanol dipping solutions of tungsten                  peroxyester derivative, with and without oxalic acid                          dihydrate. The films on ITO were colored in 0.01N                             H.sub.2 SO.sub.4 using Ag/AgCl as the reference electrode.                    Dipping                                                                              Firing    Color (Seconds)                                                                             Bleach (Seconds)                               Solution                                                                             Temp. °C.                                                                        T.sub.50                                                                              T.sub.90                                                                              T.sub.50                                                                            T.sub.90                               ______________________________________                                        No oxalic                                                                            250       44      201     15    48                                     acid                                                                          dihydrate                                                                     5 wt % 250       13      46       3     6                                     oxalic                                                                        acid                                                                          dihydrate                                                                     No oxalic                                                                            350       T.sub.50 >>600                                                                              Slow to bleach                                 acid                                                                          dihydrate                                                                     5 wt % 350       17      51      61    41                                     oxalic                                                                        acid                                                                          dihydrate                                                                     ______________________________________                                         T.sub.50 = time for the coating transmission to change 50% of the total       difference between the fully bleached and fully colored states.               T.sub.90 = time for the coating transmission to change 90% of the total       difference between the fully bleached and fully colored states.          

EXAMPLE 2

Two spin coating solutions of tungsten peroxyester derivative wereprepared as follows: (A) 25.8 grams of tungsten peroxyester derivativewere dissolved in 100 milliliters of ethanol; (B) 37.1 grams of tungstenperoxyester derivative and 3.65 grams of oxalic acid dihydrate weredissolved in 100 milliliters of ethanol. The solutions were spin-coatedonto ITO substrates. The sample (from solution A) without oxalic aciddihydrate was heated in ambient atmosphere to 250° C. for about 60minutes at 5° C. per minute. The sample with the oxalic acid dihydratedipped from solution B was pretreated as described in Example 1 and asillustrated in FIG. 1 prior to firing in ambient atmosphere to 250° C.Using solution A, crack-free coatings were obtained up to a thickness of2,200 angstroms, whereas solution B gave crack-free coatings up to 3,700angstroms.

EXAMPLE 3

Two dipping solutions of the tungsten peroxyester derivative wereprepared by adding respectively 11 grams and 31 grams of the tungstenperoxyester derivative to 70 milliliters of ethanol and adding fiveweight percent oxalic acid dihydrate. An ITO-coated substrate was dippedinto the lower concentration dipping solution and withdrawn at a rate of27 centimeters per minute. The substrate was then divided in half.One-half was pretreated in a humid atmosphere as described in Example 1,and then heated to 250° C. for about 60 minutes under ambientatmosphere. The second half of the coated substrate was also heated, butwithout the humidity pretreatment step, to 250° C. for 60 minutes. Bothcoated substrates after the 250° C. firing had a 1,000 angstrom WO₃coating which were crack-free and which had excellent electrochromicproperties. The experiment was repeated exactly for the higherconcentration dipping solution. In this case, after the 250° C. firing,the WO₃ coatings were 5,000 angstroms thick. However, the coating thathad the humidity pretreatment step was crack-free and had excellentelectrochromic properties, while the coating without the pretreatmentstep was substantially cracked and flaked off the substrate surface.

EXAMPLE 4

Forty (40) grams of tungsten peroxyester derivative and 7.69 grams ofoxalic acid dihydrate were dissolved in 70 milliliters of ethanol. ASnO₂ -coated glass substrate (10 ohm/square) obtained under the tradename TEC 10™ from Libby Owens Ford, Toledo, Ohio was dipped into thesolution and withdrawn at a rate of 46 centimeters per minute. Thecoating was pretreated as described in Example 1 and then heated inambient atmosphere to 350° C. to give a crack-free coating which was1.021 micrometers thick.

EXAMPLE 5

Thirty-one (31) grams of tungsten peroxyester derivative was dissolvedin 80 milliliters of ethanol to form the dipping solution. Oxalic aciddihydrate was added to individual dipping solutions in the followingconcentrations:

    ______________________________________                                                       molar ratio                                                    wt %           oxalic acid:tungsten peroxyester                               oxalic acid dihydrate                                                                        derivative                                                     ______________________________________                                        0.0            0.0                                                            0.5            0.092                                                          1.0            0.184                                                          2.0            0.369                                                          5.0            0.922                                                          10.0           1.84                                                           ______________________________________                                    

ITO-coated glass substrates were dipped into the solutions and withdrawnat a rate of 27 centimeters per minute. The resulting coatings werepretreated as described in Example 1 and then heated to 250° C. andtheir density determined using Rutherford Backscattering Analysis (seeFIG. 2). As can be seen from this figure, adding oxalic acid dihydrateto the dipping solution allows one to control the density of theheat-treated film.

EXAMPLE 6

Dipping solutions were prepared as described in Example 1 and to thesesolutions were added the following organic acids in the molar ratio of0.875 moles acid per mole of tungsten peroxyester derivative: oxalicacid dihydrate, malonic acid, succinic acid, glutaric acid, and adipicacid. These acids have the following melting points: oxalic - about104°-106° C., malonic - about 135°-137° C., succinic - about 187°-189°C., glutaric - about 95°-98° C. and adipic - about 152°-154° C. Thesolutions were spin-coated onto ITO substrates at speeds such thatcoating thickness after pretreating as illustrated in FIG. 1 and firingto 350° C. in air was between 2,000 angstroms and 2,300 angstroms. Thefilms were colored in 0.1N H₂ SO₄ as in Example 2 and the results aresummarized in Table 2. It was found that solutions with a more preferredmoiety, oxalic acid, yielded crack-free films whereas those with highermolecular weight organic acids were cracked at lower thicknesses (lessthan 3,000 angstroms).

                  TABLE 2                                                         ______________________________________                                        Tabulated data for the coloring kinetics at 550 nanometers                    of WO.sub.3 films from films produced with various organic                    acids. The films on ITO were colored in 0.1N H.sub.2 SO.sub.4                 using Ag/AgCl as the reference electrode.                                            Carbon  Bleached    Colored   Color Time                               Acid   Atoms   Transmittance                                                                             Transmittance                                                                           (Sec)                                    ______________________________________                                        *Oxalic                                                                              2       95.0        35.2      220                                      Malonic                                                                              3       83.0        68.1      245                                      Succinic                                                                             4       84.5        56.6      250                                      Glutaric                                                                             5       86.1        71.2      230                                      Adipic 6       85.7        72.6      200                                      ______________________________________                                         *dihydrate                                                               

EXAMPLE 7

Tungsten peroxyester derivative solution was prepared as described inExample 1, except that two weight percent glycerol was added instead ofoxalic acid. Glycerol has a boiling point of about 290° C. at 760 torr.The solution was spin-coated onto ITO-coated glass slides at spin ratesof 3,000, 2,000 and 1,000 rpm and pretreated as described in Example 1and heated to 250° C. The resulting coatings crack-free were 3,715,4,360 and 6,400 angstroms thick.

EXAMPLE 8

Tungsten peroxyester derivative solution was prepared as described inExample 1, except that two weight percent formamide was added instead ofoxalic acid dihydrate. Formamide has a boiling point of about 210° C. at760 torr. ITO-coated glass substrates were spin-coated with the solutionat spin rates of 3,000, 2,000 and 1,000 rpm respectively and pretreatedas described in Example 1 and heated to 250° C. The resulting coatingscrack-free were 3,550, 4,290 and 6,000 angstroms thick.

EXAMPLE 9

Three tungsten peroxyester derivative solutions were prepared asdescribed in Example 3, except that the solvents used were methanol,2-butanol and tetrahydrofuran. Methanol has a boiling point of about 65°C. at 760 torr. 2-butanol has a boiling point of about 99°-100° C. at760 torr. Tetrahydrofuran has a boiling point of about 67° C. at 760torr. The solutions were deposited onto ITO glass slides by spin coatingat 1,250 rpm, pretreated as illustrated in FIG. 1 and heated to 250° C.The coating from the methanol solution was 7600 angstroms thick, and itsluminous transmission in the bleached state was 80.4% and in the coloredstate 12.8%. From the 2-butanol solution the coating was 3650 angstromsthick, and its luminous transmission in the bleached state was 79.6% andin the colored state 33.4%. The coating from the tetrahydrofuransolution was 8,500 angstroms thick and in the bleached state itsluminous transmission was 77.7% and in the colored state 10%.

EXAMPLE 10

Amorphous and crystalline WO₃ coatings were prepared from a dippingsolution with and without five weight percent oxalic acid dihydrate asdescribed in Example 1. The coatings were colored using Li⁺ ions in acell containing 0.01M lithium trifluromethanesulfonate in propylenecarbonate, and a reference electrode Ag/AgNO₃ (0.01M in acetonitrile).The applied potential to color was -1.5 volts and to bleach 1.0 volts.The color kinetic data at 550 nanometers for the different coatings istabulated in Table 3. As can be seen from the table for both amorphousand crystalline tungsten oxide colored by Li⁺ ions, the rate of coloringand bleaching was enhanced by the addition of oxalic acid dihydrate. Thecharge capacity for the amorphous coating with and without oxalic aciddihydrate was 1,471 C/cm³ and 1735 C/cm³, respectively, and for thecrystalline sample with oxalic acid was 1,672 C/cm³.

                  TABLE 3                                                         ______________________________________                                        Tabulated data for the coloring kinetics at 550 nanometers                    of WO.sub.3 films from ethanol dipping solutions of tungsten                  peroxyester derivative, with and without oxalic acid                          dihydrate. The films on ITO were colored in 0.01M Li                          CF.sub.3 SO.sub.3 /propylene carbonate using Ag/AgNO.sub.3 as the             reference electrode.                                                          Dipping          Color (Seconds)                                                                             Bleach (Seconds)                               Solution                                                                             Temp. °C.                                                                        T.sub.50                                                                              T.sub.90                                                                              T.sub.50                                                                            T.sub.90                               ______________________________________                                        No oxalic                                                                            250       47      154     97    316                                    acid                                                                          dihydrate                                                                     5 wt % 250       36      124     54     99                                    oxalic                                                                        acid                                                                          dihydrate                                                                     No oxalic                                                                            350       39      129     Slow to bleach                               acid                                                                          dihydrate                                                                     5 wt % 350       59      171     100   294                                    oxalic                                                                        acid                                                                          dihydrate                                                                     ______________________________________                                         T.sub.50 = time for the coating transmission to change 50% of the total       difference between the fully bleached and fully colored states.               T.sub.90 = time for the coating transmission to change 90% of the total       difference between the fully bleached and fully colored states.          

EXAMPLE 11

Thirty-one (31) grams of tungsten peroxyester derivative, 5.95 grams ofoxalic acid dihydrate and 0.178 grams of tin (IV) chloride pentahydratewas dissolved in 70 milliliters of ethanol. SnO₂ -coated glass substrate(20 ohms/square) was dipped into the solution and withdrawn at a rate of46 centimeters per minute and pretreated as illustrated in FIG. 1 andthen heated to 350° C. The coating was 8,000 angstroms thick, and wefound that addition of a tin dopant enhanced coating adhesion to the tinoxide substrate. In general, we find benefit from doping a thin filmwith a small amount of at least one of the principal metals compoundcoating being adhered to by the sol-gel deposited layer. In the bleachedstate, its luminous transmission was 84.9%, and in the colored state was3.7%. The coating was placed in 0.1N H₂ SO₄ along with a similar coatingwithout the tin doping. After eight (8) hours, the WO₃ coating developeda hazy appearance, while the tin-doped WO₃ coating was unaffected by theacid up to a period of several weeks.

EXAMPLE 12

A tungsten oxide coating doped with rhenium was prepared as follows.Eight (8) grams of rhenium metal was reacted with a 500 milliliter 50:50mixture of H₂ O₂ (30 volume percent) and acetic acid at 0° C. Themixture was allowed to react for 90 minutes and slowly warmed to roomtemperature, and reacted for an additional 24 hours. The excess H₂ O₂and acetic acid were removed under reduced pressure at 60° C. to leave ayellow liquid of rhenium peroxy acid product. This complex is a liquidwhich is soluble in ethanol and decomposes at greater than 100° C. Thecomplex was added to a tungsten peroxyester derivative dipping solutioncontaining oxalic acid dihydrate as described in Example 1 such that themolar ratio of W to Re was 129.0. The solution was spin-coated at 1,100rpm onto ITO-coated substrates and pretreated as illustrated in FIG. 1and heated to 350° C. The coating thickness was 5,740 angstroms, andwhen colored using H⁺ ions as described in Example 1, its opticaldensity was 1.21.

EXAMPLE 13

A tungsten oxide coating doped with molybdenum was prepared as follows.Ten (10) grams of molybdenum metal was slowly added to 200 millilitersof a 50:50 mixture of H₂ O₂ (30 volume percent) and glacial acetic acidat 0° C. The metal was completely oxidized after 40 minutes to give aclear solution. The mixture was filtered and evaporated to dryness togive an organic powder of a molybdenum peroxy acid product. This complexis a yellow solid which is soluble in ethanol and decomposes at greaterthan 100° C. This complex was added to a tungsten peroxyester derivativedipping solution containing oxalic acid dihydrate prepared as describedin Example 1 such that the molar ratio of W/Mo was equal to 7.9. AnITO-coated glass substrate was dipped into the solution and withdrawn ata rate of 30 centimeters per minute. The coating was pretreated asillustrated in FIG. 1 and heated to 250° C. and had a thickness of 5000angstroms. The coating was placed in a 0.1N H₂ SO₄ acid bath, along witha similar coating without molybdenum doping. After eight (8) hours, theundoped coating developed a hazy appearance, while after two weeks, themolybdenum-doped coating was unaffected by the acid.

EXAMPLE 14

Thirty-one (31) grams of tungsten peroxyester and 5.96 grams of oxalicacid dihydrate were dissolved in 70 milliliters of dry ethanol toprepare a dip solution. A conductive coating of ITO on glass was loweredinto the solution and withdrawn at a rate of nine centimeters perminute. The resulting coating was fired to 250° C. The coating thicknesswas 2,000 angstroms. The same was cut in two, and one half of the samplewas further coated by tantalum oxide by the following process. Fivemilliliters of tantalum ethoxide was added under dry nitrogen to a flaskcontaining 60 milliliters dry ethanol and 5.38 milliliters of2,4-pentanedione. To enhance hydrolysis, 0.7477 milliliters of HCl wasadded to the solution. The solution was allowed to stir for 24 hoursprior to coating. The tungsten oxide-coated substrate previouslydescribed was immersed in the solution and withdrawn at a rate of 27centimeters per minute. The coating was then heated under ambientatmosphere at a rate of 5° C. per minute to 350° C., held at temperaturefor one hour, and cooled at 5° C. per minute to room temperature. Thisresulted in a clear layer of Ta₂ O₅, 1,000 angstroms thick. Both of thesamples were cycled in 0.01N sulfuric acid with an Ag/AgCl referenceelectrode. When -0.4 volts were applied (with respect to Ag/AgCl), thefilm colored from 75% to 35% transmission in 300 seconds at 550nanometers without the tantalum overlayer, and colored from 90% to 64.8%transmission under the same conditions with the tantalum overlayer. Thefilms were then immersed in 0.1N H₂ SO₄. The film without the tantalumoxide overlayer was dissolved or removed in eight hours, whereas thecoating with the overcoat cycled without any detectable change at theend of one month. Thus, the film without the tantalum oxide overlayerremained substantially durable to the acid, whereas the coating with theovercoat cycled in the acid without any detectable change at the end ofone month.

Of course, it is understood that the above is merely a preferredembodiment of the invention and that various changes and alterations canbe made without departing from the spirit and broader aspects thereof asset forth in the appended claims. For example, it is contemplated thatother types of electrochromically-active materials and precursorstherefor could be utilized within the broader aspects of the invention.Other methods of converting such materials might be utilized. Similarly,variations in the methods for removal of the precursor solution solventand the selected moiety might be chosen. Whereas the preferredembodiment contemplates evaporation of the solvent by heating and/orevaporation in vacuum followed by removal of the moiety by furtherheating, it is conceivable that a moiety could be selected which is alsoevaporated in a vacuum, where the moiety's vapor pressure is such that asubstantially greater vacuum is required to effect its removal than isrequired to effect removal of the precursor solution solvent. Thesevariations are illustrations of changes and alterations which are withinthe broader aspects of the invention as set forth in the appendedclaims.

The exclusive property or privileges claimed are:
 1. A method of formingan electrochromic coating on a substrate having an electricallyconductive surface comprising:providing a substrate having anelectrically conductive surface; providing a solution of an (1)electrochromic precursor, which can be converted to an electrochromicmaterial, and (2) a removable moiety in (3) a carrier solvent; saidsolvent, said moiety and said electrochromic precursor being selectedsuch that said solvent is preferentially removed in a first removal stepfrom a coating of said solution, leaving a matrix film of saidelectrochromic precursor and said moiety, and said moiety ispreferentially removed from said matrix in a second removal step, priorto or during conversion of said electrochromic precursor to anelectrochromic material; coating the conductive surface of saidsubstrate with said solution to form a coated substrate having a coatingof a desired thickness; removing said solvent from said coating in afirst removal step to form an electrochromic precursor/moiety matrixfilm on said substrate; performing a second removal step to removemoiety from said matrix; and converting said electrochromic precursor toan electrochromic material to form an electrochromic coating.
 2. Amethod in accordance with claim 1 wherein the molar ratio of said moietyto said electrochromic precursor is from about 0.1:1 to about 2:1.
 3. Amethod in accordance with claim 2 wherein the molar ratio of saidorganic moiety to said electrochromic precursor is from about 0.3:1 toabout 1:1.
 4. The method of claim 1 in which said electrochromicprecursor is selected such that it is converted to an electrochromicmaterial at an elevated conversion temperature;said first removal stepbeing conducted by one of (1) evaporating said solvent in a vacuum, (2)evaporating said solvent at an elevated first temperature which is belowsaid conversion temperature at which said electrochromic precursor isconverted to an electrochromic material, or (3) a combination of both;and said second removal step being conducted by heating saidelectrochromic precursor and moiety matrix to said conversiontemperature at which said electrochromic precursor is converted to anelectrochromic material.
 5. The method of claim 4 in which saidremovable moiety is an organic moiety having a decomposition temperaturegreater than, or a vapor pressure sufficiently low at the temperature atwhich said solvent is removed, that said organic moiety and saidelectrochromic precursor form a matrix film on said substrate after saidsolvent evaporates, and said organic moiety having a decompositiontemperature lower than, or a vapor pressure sufficiently high at saidconversion temperature at which said electrochromic precursor isconverted to an electrochromic material that said organic moiety issubstantially removed from said coating before or during saidconversion.
 6. A method in accordance with claim 5 wherein said organicmoiety has a carbon content of six carbon atoms or less per molecule. 7.A method in accordance with claim 6 wherein the molar ratio of saidmoiety to said electrochromic precursor is from about 0.1:1 to about2:1.
 8. A method in accordance with claim 7 wherein the molar ratio ofsaid organic moiety to said electrochromic precursor is from about 0.3:1to about 1:1.
 9. A method in accordance with claim 7 wherein saidorganic moiety is selected from the group consisting of oxalic acid,malonic acid, succinic acid, glutaric acid, adipic acid, formamide andglycerol.
 10. A method in accordance with claim 9 wherein saidelectrochromic precursor is selected to form a metallic oxide selectedfrom the group of tungsten oxide, molybdenum oxide, manganese oxide,chromium oxide, rhenium oxide, iridium oxide, nickel oxide and mixturesthereof.
 11. A method in accordance with claim 10 wherein said carriersolvent is selected from the group consisting of methanol, ethanol,propanol, isopropanol, sec-butanol and mixtures thereof.
 12. The methodof claim 11 in which said electrochromic precursor is aperoxyester-transition metal derivative.
 13. The method of claim 12 inwhich said second removal step is conducted by heating to a temperatureof from about 250° C. to about 400° C.
 14. The method of claim 13 inwhich said second removal step is conducted by heating to a sufficientlyhigh temperature to crystallize the resulting electrochromic coating.15. The method of claim 7 in which said second removal step is conductedby heating to a temperature of from about 250° C. to about 400° C.
 16. Amethod in accordance with claim 5 wherein the molar ratio of said moietyto said electrochromic precursor is from about 0.1:1 to about 2:1. 17.The method of claim 16 in which said second removal step is conducted byheating to a temperature of from about 250° C. to about 400° C.
 18. Amethod in accordance with claim 5 wherein said second removal stepcomprises heating said coated substrate to said conversion temperatureat a rate of from about 5° C. to about 10° C. per minute.
 19. A methodin accordance with claim 18 wherein said coated substrate is held atsaid conversion temperature for about 15 to about 120 minutes.
 20. Amethod in accordance with claim 18 wherein said first removal stepcomprises heating said coated substrate from room temperature to saidfirst elevated temperature at a rate of from about 5° C. to about 10° C.per minute.
 21. The method of claim 4 in which said conversion isconducted by heating to a sufficiently high temperature to crystallizethe resulting electrochromic coating.
 22. A method in accordance withclaim 21 wherein said electrochromic precursor is selected to form ametallic oxide selected from the group of tungsten oxide, molybdenumoxide, manganese oxide, chromium oxide, rhenium oxide, iridium oxide,nickel oxide and mixtures thereof.
 23. A method in accordance with claim4 in which said said first removal step is conducted by heating in ahumid atmosphere.
 24. A method in accordance with claim 23 wherein saidfirst removal step includes holding said substrate in a humid atmosphereat about 100° C. for about one hour.
 25. A method in accordance withclaim 24 wherein said coated substrate is heated in said first removalstep from room temperature to about 100° C. at a rate of from about 5°C. to about 10° C. per minute.
 26. The method of claim 23 in which saidremovable moiety is an organic moiety having a decomposition temperaturegreater than, or a vapor pressure sufficiently low at the temperature atwhich said solvent is removed, that said organic moiety and saidelectrochromic precursor form a matrix film on said substrate after saidsolvent evaporates, and said organic moiety having a decompositiontemperature lower than, or a vapor pressure sufficiently high at thetemperature at which said electrochromic precursor is converted to anelectrochromic material that said organic moiety is substantiallyremoved from said coating before or during said conversion.
 27. A methodin accordance with claim 26 wherein the molar ratio of said moiety tosaid electrochromic precursor is from about 0.1:1 to about 2:1.
 28. Themethod of claim 27 in which said second removal step is conducted byheating to a sufficiently high temperature to crystallize the resultingelectrochromic coating.
 29. The method of claim 27 in which said secondremoval step is conducted by heating to a temperature of from about 250°C. to about 400° C.
 30. A method in accordance with claim 4 wherein saidelectrochromic precursor is selected to form a metallic oxide selectedfrom the group of tungsten oxide, molybdenum oxide, manganese oxide,chromium oxide, rhenium oxide, iridium oxide, nickel oxide and mixturesthereof.
 31. The method in accordance with claim 30 in which saidprecursor is formed by reacting a first transition metal with hydrogenperoxide and an organic acid to form a first metal-peroxy acid product,which is then reacted with a lower alcohol to form a firstperoxyester-transition metal derivative; and by reacting a secondtransition metal with hydrogen peroxide and an organic acid to form asecond transition metal-peroxy acid product, which is then mixed withsaid first peroxyester-transition metal derivative.
 32. The method inaccordance with claim 31 which further comprises the step of reactingsaid second transition metal-peroxy acid product with a lower carbonalcohol to form a second peroxyester-transition metal derivative priorto mixing with said first peroxyester-transition metal derivative. 33.The method of claim 1 which additionally includes covering saidelectrochromic coating with a layer of tantalum pentoxide, a protonconductor on a layer which is a lithium conductor.
 34. A method inaccordance with claim 33 wherein the molar ratio of said moiety to saidelectrochromic precursor is from about 0.1:1 to about 2:1.
 35. Themethod of claim 34 in which said electrochromic precursor is selectedsuch that it is converted to an electrochromic material at an elevatedconversion temperature;said first removal step being conducted by one of(1) evaporating said solvent in a vacuum, (2) evaporating said solventat an elevated first temperature which is below said conversiontemperature at which said electrochromic precursor is converted to anelectrochromic material, or (3) a combination of both; and said secondremoval step being conducted by heating said electrochromic precursorand moiety matrix to said conversion temperature at which saidelectrochromic precursor is converted to an electrochromic material. 36.The method of claim 35 in which said removable moiety is an organicmoiety having a decomposition temperature greater than, or a vaporpressure sufficiently low at the temperature at which said solvent isremoved, that said organic moiety and said electrochromic precursor forma matrix film on said substrate after said solvent evaporates, and saidorganic moiety having a decomposition temperature lower than-, or avapor pressure sufficiently high at said conversion temperature at whichsaid electrochromic precursor is converted to an electrochromic materialthat said organic moiety is substantially removed from said coatingbefore or during said conversion.
 37. A method in accordance with claim36 wherein said organic moiety has a carbon content of six carbon atomsor less per molecule.
 38. An electrochromic coating on a substratehaving an electrically conductive surface formed by:providing asubstrate having an electrically conductive surface; providing asolution of an (1) electrochromic precursor, which can be converted toan electrochromic material, and (2) a removable moiety in (3) a carriersolvent; said solvent, said moiety and said electrochromic precursorbeing selected such that said solvent is preferentially removed in afirst removal step from a coating of said solution, leaving a matrixfilm of said electrochromic precursor and said moiety, and said moietyis preferentially removed from said matrix in a second removal step,prior to or during conversion of said electrochromic precursor to anelectrochromic material; coating the conductive surface of saidsubstrate with said solution to form a coated substrate having a coatingof a desired thickness; removing said solvent from said coating in afirst removal step to form an electrochromic precursor/moiety matrixfilm on said substrate; performing a second removal step to removemoiety from said matrix; and converting said electrochromic precursor toan electrochromic material to form an electrochromic coating.